The present invention relates to a process for isomerizing C.sub.8 alkylaromatic hydrocarbons.
Processes for producing particular xylene isomers from C.sub.8 alkylaromatic feedstocks are well known. Typically, a selected xylene isomer is received from a petroleum fraction, such as reformate, rich in C.sub.8 alkylaromatics, as by fractionation, crystallization, or molecular sieve-type separation. Paraxylene is usually the isomer sought to be recovered, orthoxylene is occasionally the selected isomer, and metaxylene and ethylbenzene are rarely desired as products. After the selected xylene isomer has been removed from the petroleum fraction, the C.sub.8 alkylaromatic residue is often treated in a C.sub.8 alkylaromatic isomerization operation in order to form additional amounts of the selected xylene isomer. The newly formed amounts of the xylene isomer are then recovered from the isomerate by the same separation operation used with the original petroleum fraction. Usually, the isomerization operation involves primarily conversion of metaxylene, and sometimes ethylbenzene, to paraxylene. Ethylbenzene has been found to be relatively difficult to convert to xylenes, as compared to the relatively easy conversion of one xylene isomer to another. The concentration of ethylbenzene in an isomerization-separation system tends to build up undesirably in the processing streams, as the selected xylene isomer is removed and the residue is recycled to isomerization.
Various isomerization catalysts and flow schemes have been suggested by the art in attempting to provide efficient isomerization and recovery systems for producing a selected xylene isomer. For example, U.S. Pat. No. Re. 25,753 discloses a two-stage process for isomerizing xylenes. In the first stage, a xylene, or non-equilibrium mixture of xylenes, is contacted with a hydrogenation-dehydrogenation catalyst under hydrogenation conditions to convert a large proportion (10-35%) of the xylenes in the feed to naphthenes. In the second stage, the naphthenes produced in the first stage are contacted with a hydrogenation-dehydrogenation catalyst under dehydrogenation conditions to reconvert the naphthenes to xylenes, and simultaneously to isomerize the xylenes during dehydrogenation. One catalyst described as useful in the process is platinum on alumina or silica-alumina.
U.S. Pat. No. 3,078,318 describes the isomerization of a xylene or non-equilibrium mixture of xylenes with a platinum-halogen-alumina catalyst in a hydrogen atmosphere at 700.degree.-1100.degree. F and 1-1500 atmospheres pressure. A selected xylene isomer is separated from the isomerization reactor effluent and the residue from the isomer separation step is recycled to the isomerization step.
U.S. Pat. No. 3,381,048 describes a process for isomerization of a xylene isomer or non-equilibrium mixture of xylene isomers using a platinum-halogen-alumina catalyst. In the process, the water content of the hydrocarbon feed to the isomerization reactor is kept at 20-200 parts per million.
U.S. Pat. No. 3,538,173 describes a process for isomerizing xylenes in which ethylbenzene in a C.sub.8 alkylaromatic-containing stream is isomerized to xylenes by controlling the C.sub.8 naphthenes content in the feed introduced into the isomerization reactor to keep the C.sub.8 naphthenes content of the feed at 2-9 weight percent of the C.sub.8 alkylaromatic content of the feed. A platinum-halogen-alumina catalyst is employed in the isomerization reactor at a temperature of 700.degree.-840.degree. F and a pressure of 3-20 atmospheres.
U.S. Pat. No. 3,553,276 describes a process for isomerizing xylenes in which, during recovery of a selected xylene isomer from the reactor effluent, loss of C.sub.8 naphthenes from the system is minimized by maintaining a high concentration of diluent toluene in the effluent from the isomerization reactor. This is accomplished by introducing large amounts of diluent toluene into the isomerization reactor in the feed. A platinum-halogen-alumina catalyst is used in the isomerization step at a temperature of 32.degree.-1290.degree. F and a pressure of 1-100 atmospheres, or more.
U.S. Pat. No. 3,879,484 describes a process for isomerizing C.sub.8 alkylaromatic hydrocarbons such as xylenes by contacting the C.sub.8 alkylaromatic with a platinum-rhenium-halogen-alumina catalyst at a temperature of 32.degree.-1112.degree. F and a pressure of 1-100 atmospheres.